drm/amdgpu: Only add rqs for initialized rings.

I don't see another way to figure out if a ring is initialized if
the hardware block might not be initialized.

Entities have been fixed up to handle num_rqs = 0.

Signed-off-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>